Transmitter circuits are often used in wireless communication devices to transmit data signals via a suitable antenna in the form of electromagnetic radiation. “Wireless communication devices” include in general any device capable of connecting wirelessly to a network, and in particular mobile devices including mobile or cell phones (including so-called “smart phones”), personal digital assistants, pagers, tablet and laptop computers, content-consumption or generation devices (for music and/or video for example), data cards, USB dongles, etc., as well as fixed or more static devices, such as personal computers, game consoles and other generally static entertainment devices, various other domestic and non-domestic machines and devices, etc.
In order to emit a signal with a suitable power and range, a power amplifier is often used to amplify the signal prior to transmission. Power amplifiers typically operate more efficiently when near the limit of their operating range. This observation has led to the development of transmitters that dynamically adjust the supply voltage of a power amplifier in dependence on properties of the signal being transmitted. One such type of transmitter arrangement, commonly referred to in the art as an envelope tracking transmitter, dynamically adjusts the supply voltage of a power amplifier in dependence on the magnitude (or “envelope”) of the signal being amplified.
FIG. 1 shows schematically an example of an envelope tracking transmitter arrangement 100 as known in the art. Data signal 102 is supplied to modulation circuit 104 which performs the necessary operations to modulate the data signal onto a higher frequency carrier signal. The modulated signal is then supplied to power amplifier 106 in order to amplify the strength of the signal prior to transmission via antenna 108. In order to improve the efficiency of the transmitter 100, an envelope detector 110 is used to determine the magnitude of data signal 102. The determined magnitude is then referenced against previously stored data in the form of shaping table 112, detailing a predetermined mapping between the magnitude of the signal and an efficient supply voltage for power amplifier 106. Hence, the supply voltage of power amplifier 106 is dynamically altered during operation to improve the operating efficiency of transmitter 100.
However, properties of the transmitter components (including the gain of the power amplifier) may change over time due to e.g. component ageing, temperature effects etc. and the mapping stored in the shaping table may no longer reflect a suitably efficient relationship. A possible method for ongoing calibration of an envelope tracking transmitter is to monitor the gain of the power amplifier to detect any deviations for the expected gain. However, detection of small changes in gain is difficult during normal operation of a transmitter for several reasons. For example, a transmitter typically comprises multiple amplifier stages, and the propagation of gain tolerances through each of these stages makes the total gain of all stages hard to establish to a sufficient degree of accuracy.
Hence, it would be desirable to provide improved measures for calibrating a transmitter arrangement where the supply voltage of an amplification stage is varied, in particular for ongoing calibration during normal operation of the device.